FIG. 9 shows an overview of the complete DAB system. Such a system comprises an audio encoder 1, a convolutional encoder 2, a time interleaving circuit 3, a circuit to generate a fast information channel with a TII database 4, a multiplexer 5, a frequency interleaving circuit 6, a phase reference symbol generator 7, a null symbol generator plus TII generating circuit 8, a multiplexer 9, an IFFT circuit 10, a D/A-converter 11, and an RF transmitter 12 on a sender side to transmit audio data and information data over a channel 13, and an RF receiver 14, a A/D-converter 15, a FFT circuit 16, a synchronization circuit 17, a TII detection circuit 18, a demodulation circuit 19, a deinterleaving circuit 20, a Viterbi decoder 21 and an audio decoder 22 to retrieve the audio data and information data from the channel 13 on the receiver side. These components are connected and work in a well-known fashion. The present invention only concerns the TII detection as it takes place in the TII detection circuit 18, therefore, the following description will only be related thereto.
According to the ETS 300 401 standard the DAB stream starts with a so-called null symbol followed by a so called TFPR-Symbol for the receiver synchronization. The null symbol is also defined to carry a TII signal. Each transmitter in the single frequency network is assigned a main id and a sub id for unique identification. This identification is mapped to a certain pattern with 16/8/4/2 set carrier pairs in the spectrum of the null symbol according to the DAB modes I-IV. Based on mode II which has 384 valid carriers a so called comb block is defined. For modes I and IV this block is repeated 4 and 2 times, respectively. For mode III only a half block is available. This pattern is transmitted every 2nd DAB frame in the null symbol spectrum. The set carriers have to be detected and the respective main and sub ids have to be calculated. Additionally thereto, the complete list of all main and sub ids available in a single frequency network are transmitted in a fast information channel, i.e. FIC, of the date stream. With the help of TII the receiver can filter automatically local information from the data stream.
FIG. 11 shows the spectrum of a null symbol including TII of the incoming DAB stream in the receiver. The spectrum shown is transmitted in DAB mode I where 4 comb blocks are available. This means that the set TII pairs are transmitted four times within every second null symbol.
The construction of the TII was also defined with the regard to a possible navigation. The use of neighbouring carrier pairs allows the estimation of the propagation delay by evaluating their phase difference. If three delays are known from the reception of three transmitters, i.e. three TII codes, a localisation of the mobile receive is possible with hyperbolic navigation.
In a Diploma thesis "Sendererkennung im Gleichwellennetz" by Petra Stix made for Sony Deutschland GmbH and University Stuttgart, Institut fur Nachrichtenubertragung, the following method to detect TII in a DAB stream as shown in FIG. 10 is published.
First, in a step P1, the spectrum S(.omega.) of a null symbol including TII, as it is shown in FIG. 11, is derived. In the next steps P2 and P3, the absolute value of the complex amplitudes of the four equal comb blocks transmitted in said symbol are added, because only the amplitudes of the TII carriers must be detected and the single phases of the carriers are not relevant for this detection. Herewith, the signal power is increased in comparison to the noise, if the signal is above the noise level. Thereafter, in step P4, two neighbouring carriers are added, since always carrier pairs are set for TII and therewith the signal power is increased again. Before the set carriers are decoded to main and sub ids in steps P9 and P10, a decision has to be made if a respective carrier is set in step P5. Therefore, a threshold is necessary. This threshold is derived from the noise power in the spectrum in the left and right of the DAB block in step P6 that gets multiplied with the number of TII frequency blocks in step P7 and with 2 in step P8, before being used to determine whether a carrier is set or not in step P5.
This method for deciding if there is a certain carrier set fails at low signal-to-noise ratios, not at last because the method for determining the threshold is practically not useful due to the spectrum shape in the receiver, as it is shown in FIG. 11. Further, the error of the estimated propagation delays at low signal-to-noise ratios rises exponentially so that a navigation or localisation is very inaccurate.